1. Field of the Invention
The present invention relates to an image pickup apparatus comprising an image sensor such as CCD or CMOS image sensor, and particularly to an image pickup apparatus with a back focus adjustment mechanism for adjusting a distance (back focus) from a lens to the image sensor.
2. Description of Related Art
In a monitoring camera with interchangeable lenses (CS mount), a lens unit including a lens is detachably attached to a lens mount section of a camera body which is provided therein with an image sensor such as a CMOS image sensor, and the lens unit is exchangeable. The camera of this kind is equipped with a back focus adjustment mechanism for adjusting back focus when exchanging the lens unit or the like.
When photographing in daytime using a camera, the light taken in through the lens includes visible light and near infrared rays. Since the near infrared component makes images reddish, the near infrared component needs to be removed from the light taken in through the lens. In contrast, when photographing in nighttime, the light taken in through the lens includes almost no visible light, and therefore, the near infrared component needs to be taken in to enhance sensitivity.
Therefore, a sliding member which is slidable in a direction perpendicular to an optical axis of the image sensor is arranged in front of the image sensor, and the sliding member is provided with an infrared blocking filter and a dummy glass plate arranged next to each other along its sliding direction. When photographing in daytime, the sliding member is moved to a day photographing position where the infrared blocking filter is positioned on the optical axis of the image sensor, and when photographing in nighttime, the sliding member is moved to a night photographing position where the dummy glass plate is positioned on the optical axis of the image sensor.
In such a day and night switching camera, when photographing in daytime, the light taken in through the lens passes through the infrared blocking filter before entering the image sensor, and when photographing in nighttime in contrast, the light taken in through the lens passes through the dummy glass plate before entering the image sensor. However, since wavelength of light which enters the image sensor is different between day and night, focal point of the lens is different between daytime photographing and nighttime photographing, resulting in focus displacement. Therefore, the dummy glass plate is designed so as to have a thickness with which optical path length extension equal to that with the infrared blocking filter is obtained.
For example, in the case of employing an infrared blocking filter having a thickness of 2.78 mm and a dummy glass plate having a thickness of 2.88 mm, an optical path length extension λ′ with the infrared blocking filter is obtained as follows, where refraction index n1 is 1.5443.λ′=(1−1/n1)*2.78=0.9798 mm
Whereas, an optical path length extension λd′ with the dummy glass plate is obtained as follows, where refraction index n2 is 1.5163.λd′=(1−1/n2)*2.88=0.980 mm
Therefore, the optical path length when the infrared blocking filter intervenes in the optical path and the optical path length when the dummy glass plate intervenes in the optical path are equivalent, and the displacement of imaging plane position due to switching the photographing modes is negligible.
As shown in FIG. 17a, when photographing in daytime, the light which has passed a lens 20 further passes an infrared blocking filter 671 and forms an image on an image surface B1. In contrast, as shown in FIG. 17b, when photographing in nighttime, the light which has passed the lens 20 further passes a dummy glass plate 672 and forms an image on an imaging plane B2. As described above, by adjusting the thickness of the dummy glass plate 672, the imaging plane 32 in the nighttime photographing mode is set at generally the same position as that of the imaging plane 31 in the daytime photographing mode.
Although the imaging plane B2 in the nighttime photographing mode is set at generally the same position as that of the imaging plane 31 in the daytime photographing mode as described above, since there are assembly errors or the like in an actual camera, when the daytime photographing mode is switched to the nighttime photographing mode, back focus is adjusted by a back focus adjustment mechanism to correct deviation from the back focus in the daytime photographing mode (deviation of optical path length).
In back focus adjustment in a conventional camera at the time of switching modes, as shown in FIG. 16 for example, first in step S11, back focus is changed to move the imaging plane position from an origin position (an imaging plane position set in back focus adjustment in the daytime photographing mode) to a FAR-side end point (an end point in a variable range of the back focus farther from the lens). In this process, an AF evaluation value (high frequency component of an image signal output from a CMOS image sensor) is sampled while correlated with a rotation angle of a drive motor at a constant period.
Subsequently in step S12, the back focus is changed to move the imaging plane position from the FAR-side end point to a NEAR-side end point (an end point in a variable range of the back focus nearer to the lens) via the origin point. In this process, an AF evaluation value is sampled while correlated with a rotation angle of a drive motor at a constant period.
And then in step S13, the back focus is changed to move the imaging plane position from the NEAR-side end point to an imaging plane position where the maximum AF evaluation value among sampled AF evaluation values is obtained, and the back focus adjustment procedure ends.
However, in a case where, for example, the distance from the FAR-side end point to the NEAR-side end point is 1 mm and moving speed of the imaging plane position is 0.1 mm/s, it takes 5 seconds to complete step S11 of FIG. 16, 15 seconds to complete step S12, and approximately 20 seconds to complete step S13. Thus, there is a problem of taking as long as approximately 20 seconds for back focus adjustment.